1. Field of the Invention
The present invention generally relates to plasma display apparatuses, and particularly relates to a plasma display apparatus having an improved display contrast.
2. Description of the Related Art
Plasma display panels have two glass plates on which electrodes are formed, and discharge-purpose gas fills the gap of the order of 100 microns between the two glass plates. Voltages higher than the discharge threshold voltage are applied between the electrodes to start gas discharge, and ultraviolet light generated from the discharge induces the light emission of photo florescent provided on the plate, thereby effecting screen displaying.
FIG. 1 is a diagram showing a schematic configuration of a plasma display apparatus.
A display panel 10 includes first electrodes 14 and second electrodes 15 disposed in parallel, and further includes third electrodes 16 disposed in perpendicular thereto. The first electrodes 14 and the second electrodes 15 are used to provide sustain discharge for display-purpose light emission. Voltage pulses are applied between the first electrodes 14 and the second electrodes 15, thereby carrying out sustain discharge. Either one of the first electrodes 14 and the second electrodes 15 serve as scan-purpose electrodes for writing display data. The third electrodes 16 are used to select display cells 17 that are to emit light. A voltage for writing discharge is applied between the third electrodes 16 and either one of the first electrodes and the second electrodes, so as to select discharge cells. The first electrodes 14, the second electrodes 15, and the third electrodes 16 are connected to a first driving circuit 11, a second driving circuit 12, and a third driving circuit 13, respectively, which serve to generate voltage pulses for specific purposes.
FIG. 2 is a drawing showing details of the display panel unit 10 of the apparatus shown in FIG. 1.
The first electrodes 14 serving as X electrodes and the second electrodes 15 serving as Y electrodes are laid out in parallel. Electrodes for display lines L1 through L4 are only shown in this figure. The third electrodes 16 serving as address electrodes are further formed together with shields 18 for separating the discharge cells. Details of discharge operations will be described later.
FIG. 3 is a drawing showing a frame configuration for explaining driving sequences.
Discharge of a plasma display panel can only assume either one of the xe2x80x9conxe2x80x9d state and the xe2x80x9coffxe2x80x9d state, so that the density, i.e., the gray scale, is represented by the number of repeated light emissions. In order to efficiently implement this, a frame is divided into 10 sub-fields, for example. Each sub-field is comprised of a reset period, an address period, and a sustain discharge period. During the reset period, all cells are equally initialized regardless of lighting status in the previous sub-fields, e.g., are placed in the condition in which wall charge is erased. During the address period, selective discharge (addressing discharge) is performed to select the on/off states of cells in accordance with the display data, thereby generating wall charge that places cells in the xe2x80x9conxe2x80x9d state. During the sustain discharge period, discharge is repeated in the cells where addressing discharge was performed, thereby emitting light. The length of the sustain discharge period, i.e., the number of repeated light emissions, differs from sub-field to sub-field. For example, the number of repeated light emissions may be determined such that ratios between sub-fields from the first sub-field to the tenth sub-field are 1:2:4:8: . . . :512. Sub-fields are selected in accordance with the luminance level of the display cell so as to be subjected to gas discharge, thereby achieving a desired gray scale display.
FIG. 4 is a drawing showing the way the reset discharge emits light.
When xe2x80x9cblackxe2x80x9d is displayed in plasma display panels, it is desirable not to have any electrical discharge. Under the conditions where almost no ions, metastable atoms, or the like are present in the cell space, however, addressing discharge may not take place even when the required voltage is applied between the electrodes. In order to avoid this, all cells are periodically subjected to gas discharge.
There are two methods for such periodic discharge. One is to carry out discharge stronger than a predetermined intensity at the time of a start of the first sub-field, as shown in FIG. 4, (a). The other is to carry out small-scale discharge during the reset periods of all the sub-fields, as shown in FIG. 4, (b). These methods can provide a darkroom contrast of about 300:1 to 600:1. To be specific, the brightness of a black portion will be less than 1 cd/m2. Further, these two methods may be combined such that the resetting of a small-scale light emission is performed once in each frame or once in each field. In this case, a darkroom contrast of about 3000:1 is achieved. However, a complete darkness cannot be obtained, and an issue of stable operation still remains to be addressed.
FIG. 5 is a drawing showing waveforms for driving the first sub-field (e.g., SF1 in FIG. 4, (a)) of a given frame.
During the reset period, a voltage, e.g., 300 V (Vw of FIG. 5, (b)), higher than the discharge threshold voltage is applied as a pulse to the X electrodes. This pulse causes gas discharge at all the cells regardless of the lighting status thereof in the preceding sub-field, thereby creating wall charge. When this pulse is gone, discharge starts again because of a voltage generated by the wall charge. Since no voltage difference exists between the electrodes, however, space charge generated by the discharge is neutralized to create the uniform condition of no wall discharge. Although most of the charge is neutralized, some ions and metastable atoms remain in the discharge space, serving as seeds to reliably generate addressing discharge. This is generally referred to as a seeds effect or priming effect.
During the address period after this, a scan pulse (Vy of FIG. 5, (c)) is applied to the Y electrodes serving as scanning electrodes, and address pulses (Va of FIG. 5, (a)) are applied to the address electrodes of the cells that are to emit light, thereby effecting gas discharge. This discharge spreads to the space on the side of the X electrodes, thereby generating wall charge between the X electrodes and the Y electrodes. This scanning is performed with respect to all the display lines.
During the sustain discharge period, sustain pulses of a voltage Vs (about 170 V) are repeatedly applied. At the cells where wall charge is in place by the addressing discharge, the sustain pulse voltage is added to the voltage of wall charge, thereby exceeding the discharge threshold voltage and starting actual discharge. At the cells where no addressing discharge was performed, no discharge is initiated since there is no wall charge.
FIG. 6 is a drawing showing waveforms for driving a sub-field during which no reset discharge of FIG. 5 is performed.
The sub-field shown in FIG. 6 corresponds to SF1 through SF10 of FIG. 4, (b). During the reset period, an erase pulse of a voltage Vb (FIG. 6, (b)) having a gentle slope is applied to all the cells. This causes discharge at the cells that emitted light in the preceding sub-field, thereby erasing wall discharge. Operations during the address period and the sustain discharge period are the same as those of FIG. 5.
FIG. 7 is a drawing showing another configuration of a display panel unit different from that of FIG. 2.
In a display panel unit 10A of FIG. 7, X electrodes and Y electrodes serving as display electrodes are provided in turn at equal intervals so as to cross address electrodes A1 through A4. All gaps between the electrodes are utilized as display lines (L1, L2, . . . ). This configuration is called an ALIS (alternate lightning of surfaces) method, and is disclosed in Japanese Patent No. 2801893. Since all the gaps between the electrodes are utilized as display lines, the number of electrodes is half as many as that of FIG. 2, which provides a basis for a cost reduction and a scale reduction.
FIG. 8 is a drawing showing the principle of light emission of the ALIS (alternate lightning of surfaces) method.
Since all the gaps between electrodes serve as display lines, it is impossible to light up all the display lines simultaneously. Lighting of odd-number lines and even-number lines are temporally separated to effect displaying.
FIG. 9 is a drawing showing a frame configuration of the ALIS method.
One frame is divided into two fields, each of which is comprised of a plurality of sub-fields. The first field is used for the displaying of odd-number lines, and the second field is used for the displaying of even-number lines.
FIG. 10 is a drawing showing driving signal waveforms of the ALIS method.
Details of operations of the ALIS method are disclosed in the Japanese Patent Laid-open Application No. 2000-075835. During the reset period, a pulse having a gentle slope (Vwy of FIG. 10, (c) and (e)) is used to generate weak writing discharge, and a subsequent pulse (xe2x88x92Vey of FIG. 10, (c) and (e)) is used to generate erase discharge. These discharges are weak so as to suppress the intensity of light emission. Because of this, even when all the cells are subjected to this reset discharge in all the sub-fields, the luminance level of the black level does not increase. This corresponds to the configuration of FIG. 4, (b).
As described above, the luminance level of black display can be suppressed to some extent by carefully designing driving signal waveforms and sequences. A contrast ratio achieved to date in the darkroom is about 300:1 to 600:1 or 3000:1. Further, a white luminance of a small area is about 500 cd/m2. When the display apparatus is actually used, an optical filter having a transparency rate of 50 to 60% is situated in front of the panel with an aim of avoiding a contrast reduction in a bright room caused by light reflection on the panel surface. Even when the panel of itself achieves 500 cd/m2, the luminance level after passing through the filter is reduced to less than 300 cd/m2. Television sets using commercially available CRTs have a peak luminance level of about 500 cd/m2, so that plasma displays need to be improved to achieve higher luminance levels. To this end, various photo florescent materials that can achieve higher luminance levels are developed and used in practice. This, however, results in an increase in the luminance level of the black level. If the darkroom contrast is 500:1 with a filter attached to the panel, and the peak luminance level is 500 cd/m2, then, the black-level luminance level is 1 cd/m2. When seeing movies or the like in conditions close to the darkroom, even the luminance level of 1 cd/m2 appears to be rather bright, resulting in a degradation of display quality. In the case of CRTs, a luminance level almost equal to 0 cd/m2 is now available, so that the same level of blackness is expected for plasma display apparatuses as well.
FIG. 11 is a drawing showing relationships between externally provided video signals and operations of a related-art plasma display panel.
FIG. 11 shows (a) frames, (b) a vertical synchronizing signal (Vsync), (c) display data, (d) reset discharge, (e) display status, and (f) light emission conditions.
Data (display data of FIG. 11, (c)) of one frame that constitutes one image screen is supplied each time a vertical synchronizing signal (Vsync of FIG. 11(b)) corresponding to one frame is supplied. Data of one frame of a video signal is stored in a storage device (memory) of the apparatus. During a next vertical synchronization period, display data is read from the memory with respect to each sub-field, and is supplied to the driving circuitry to drive the panel. As shown in FIG. 11, (d) through (f), even when the black level having no image data al all is to be displayed, the reset discharge is carried out for each Vsync, so that a luminance of some level is inevitably observed.
Accordingly, there is a need for a plasma display apparatus that reduces the black level luminance level as much as possible.
It is a general object of the present invention to provide a method and an apparatus that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a method and an apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of driving a plasma display panel that includes the steps of detecting, with respect to each cell, whether display data is present, avoiding reset discharge with respect to a cell that is to display a black level because of absence of the display data, and generating reset discharge prior to displaying of the display data with respect to a cell that is to display a non-black level because of presence of the display data.
In the invention as described above, the cells that displays the black level continuously is not subjected to reset discharge, whereas the cells that are to display a non-black level after displaying of the black level are subjected to reset discharge at the start of a frame or a field for displaying the non-black level. Namely, a check as to the presence/absence of the display data is made on a cell-specific basis, and the reset discharge is performed only with respect to a cell that has the display data according to the check. Through the operations as described above, the seeds effect is created by reliably generating reset discharge only at the cells to be used for displaying, so that stable display discharge can be achieved while suppressing light emission to zero levels in black portions. According to this method, an infinite darkroom contrast can be achieved in theory.
According to the present invention, the luminance level of the black level can be reduced relative to the related-art configuration without undermining stable operations of the panel. As a result, a darkroom contrast of 300:1 to 600:1 in the related-art configuration can be improved to 1000:1 to ∞:1.